LIGO/Virgo G298936

The LIGO Scientific Collaboration and Virgo report:

The gstlal CBC analysis (Messick et al. Phys. Rev. D 95, 042001, 2017)
identified candidate G298936 during real-time processing of data from LIGO
Hanford Observatory (H1) and LIGO Livingston Observatory (L1) at
2017-08-23 13:13:58.518 UTC (GPS time: 1187529256.518). The Virgo (V1)
detector was online but was not included in the analysis because it had
recently re-acquired lock. We are investigating the quality of the V1 data
to determine whether it can be used.

G298936 is an event of interest because its false alarm rate, as
determined by the online analysis, is 1.739e-11 Hz, or about 1 in 1800
years. The event's properties can be found at this URL:

https://gracedb.ligo.org/events/G298936

This event was also identified in real-time by two other CBC pipelines,
PyCBC (Nitz, et al. 2017, arXiv:1705.01513
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 ) and MBTA (Adams et al. CQG 33,
175012, 2016). It was also detected by the unmodeled burst pipelines, cWB
(Klimenko et al. Phys. Rev. D 93, 042004, 2016) and LIB (Lynch et al.
Phys. Rev. D 95, 104046, 2017).

The event appears consistent with the merger of two black holes, and there
is little chance either component was a neutron star. For more details on
the source classification, please consult this technical document:
https://dcc.ligo.org/T1600571/public/main .

A rapid localization with distance information generated by the BAYESTAR
pipeline (e.g., Singer et al. 2016, ApJL 829, 15) is available and can be
retrieved from the GraceDB event page: bayestar.fits.gz.

The 50% credible region spans about 605 deg2 and the 90% region about 2145
deg2. The probability is concentrated in a pair of long, thin arcs that
spread across both the northern and southern hemispheres. Marginalized
over the whole sky, the a posteriori luminosity distance estimate is 1387
+/- 414 Mpc. It may be possible to improve the localization if the V1 data
can be included.

Updates on our analysis of this event, including updated localizations
will be sent as they become available.

I. Bartos, S. Countryman (Columbia), C. Finley (U Stockholm), E. Blaufuss (U Maryland), R. Corley, Z. Marka, S. Marka (Columbia) on behalf of the IceCube Collaboration

In an analysis completed at 2017-08-23 13:41:57 UTC, we searched IceCube online track-like neutrino candidates (GFU) detected in a [-500,500] second interval about the LIGO-Virgo trigger G298936. We compared the candidate source directions of 8 temporally-coincident neutrinos to the BAYESTAR skymap, with the following parameters:

#            dt[s]     RA[deg]    Dec[deg]      E[TeV]  Sigma[deg]
------------------------------------------------------------------
1.         -398.00       226.9       -11.4        6.16         0.2
2.         -172.94        84.3        19.3        0.99         0.9
3.          -62.90         0.5         0.1        0.49         0.6
4.          155.56        36.5        39.7        0.82         0.7
5.          234.88       265.5        10.3        0.81         0.6
6.          263.89       174.4        23.7        1.61         3.4
7.          388.57       341.9       -26.9       15.35         0.2
8.          473.06       346.6        33.8        0.73         0.8


(dt--time from GW in [seconds]; RA/Dec--sky location in [degrees]; E--reconstructed secondary muon energy in [TeV]; Sigma--uncertainty of direction reconstruction in [degrees])

The analysis found NO COINCIDENT ONLINE TRACK-LIKE NEUTRINO CANDIDATES detected by IceCube within the 500 second window surrounding G298936 within the BAYESTAR skymap.

A coincident neutrino-GW skymap has been posted to GraceDB (<https://gracedb.ligo.org/apiweb/events/G298936/files/coinc_skymap_initial_icecube.png,0>). A JSON-formatted list of the above neutrinos can be downloaded from GraceDB at: <https://gracedb.ligo.org/apiweb/events/G298936/files/IceCubeNeutrinoList.json,0>

In addition, we are performing coincident searches with other IceCube data streams, including the high-energy starting events (HESE) and Supernova triggers.  HESE events have typical energies > 60 TeV and start inside the detector volume, leading to a relatively pure event sample with a high fraction of astrophysical neutrinos.  The SN trigger system is sensitive to sudden increases in photomultiplier counts across the detector, which could indicate a burst of MeV neutrinos.  We will submit separate GCN circulars if coincident HESE or SN triggers are found.

The IceCube Neutrino Observatory is a cubic-kilometer neutrino detector operating at the geographic South Pole, Antarctica.  For a description of the IceCube realtime alert system, please refer to <http://adsabs.harvard.edu/cgi-bin/bib_query?arXiv:1610.01814
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 >; for more information on joint neutrino and gravitational wave searches, please refer to <http://adsabs.harvard.edu/cgi-bin/bib_query?arXiv:1602.05411
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 >.

V. Savchenko (ISDC, University of Geneva, CH)
on behalf of the INTEGRAL group:
S. Mereghetti (IASF-Milano, Italy),
C. Ferrigno ((ISDC, University of Geneva, CH),
E. Kuulkers (ESTEC/ESA, The Netherlands),
A. Bazzano (IAPS-Roma, Italy), E. Bozzo,
T. J.-L. Courvoisier (ISDC, University of Geneva, CH)
S. Brandt (DTU - Denmark) R. Diehl (MPE-Garching, Germany)
L. Hanlon (UCD, Ireland) P. Laurent (APC, Saclay/CEA, France)
A. Lutovinov (IKI, Russia) J.P. Roques (CESR, France)
R. Sunyaev (IKI, Russia) P. Ubertini (IAPS-Roma, Italy)

The INTEGRAL spacecraft has a highly elliptical orbit and the
instruments are switched off around the perigee passage, every 2.6
days, to prevent radiation-induced damages. Unfortunately, at the time
of the LIGO/Virgo trigger G298936 (2017-08-23 13:13:58 UTC) the
spacecraft was preparing to the start the observations after the
perigee passage between the orbits number 1853 and 1854 and no
scientific instrument data are available before
2017-08-23T13:34:33.565 (~20 minutes after the event).

M. Ageron (CPPM/CNRS), B. Baret (APC/CNRS), A. Coleiro (IFIC & APC), D. Dornic (CPPM/CNRS), A. Kouchner (APC/Universite Paris Diderot), T.
Pradier (IPHC/Universite de Strasbourg) report on behalf of the ANTARES Collaboration:

Using on-line data from the ANTARES detector, we have performed a follow-up analysis of the recently reported LIGO/Virgo G298936 event
using the bayestar probability map at event time. The ANTARES visibility at the time of the alert together with the 50% and 90% contours
of the probability map are shown in: https://www.cppm.in2p3.fr/~dornic/events/G298936/visi_230817.png <https://www.cppm.in2p3.fr/~dornic/events/G298936/visi_230817.png> (gwantares/GW@ANT40). Considering 
the location probability provided by the LIGO/VIRGO collaboration, there is 50% chance that the GW emitter was in the ANTARES field of view.

No up-going muon neutrino candidate events were recorded in the ANTARES sky during a +/- 500s time-window centered on the G298936 event
time. The expected number of atmospheric background events in the region visible by ANTARES is ~1.1e-2 in the +/- 500s time window. An
extended search during +/- 1 hour gives no up-going neutrino coincidence.

The results of a second analysis covering the full-sky as well as an estimate of the upper limit on the associated neutrino fluence will be sent in a
subsequent circular.

ANTARES, being installed in the Mediterranean Deep Sea, is the largest neutrino detector in the Northern Hemisphere. It is primarily
sensitive to astrophysical neutrinos in the TeV-PeV energy range.  At 10 TeV, the median angular resolution for muon neutrinos is below 0.5 degrees.
In the range 1-100 TeV, ANTARES has the best sensitivity to a large fraction of the Southern sky.

M. Cardillo (INAF/IAPS), F. Verrecchia (SSDC and INAF/OAR), A. Bulgarelli
(INAF/IASF-Bo), M. Tavani (INAF/IAPS, and Univ. Roma Tor Vergata), C. Pittori
(SSDC and INAF/OAR), M. Pilia (INAF/OA-Cagliari), G. Piano (INAF/IAPS), A.
Ursi(INAF/IAPS), I. Donnarumma (ASI), F. Longo (Univ. Trieste and INFN Trieste),
G. Minervini, A. Argan, Y. Evangelista (INAF/IAPS), F. Lucarelli (SSDC and
INAF/OAR), N. Parmiggiani, A. Zoli, V. Fioretti, F. Fuschino (INAF/IASF-Bo),
A. Trois (INAF/OA-Cagliari) M. Marisaldi (INAF/IASF-Bo and Bergen University),
A. Giuliani (INAF/IASF-Mi), report on behalf of the AGILE Team:

In response to the LIGO/Virgo GW event G298936 (T0 = 2017-08-23 13:13:58.518
UTC; GCN #21656
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 ), analysis of AGILE data shows that the satellite at T0 was
in the South Atlantic Anomaly (SAA). Scientific telemetry was inhibited
during the time interval (T0 -300 s; T0 + 500 s).

At the moment of data acquisition obtained near the time interval defined above
we obtained preliminary 3-sigma fluence upper limits (UL) for a 1 s integration
time at different celestial positions within the G298936 90% c.l. localization
region, from a minimum of 5.1e-7 erg cm^-2 to a maximum of 7.8e-7 erg cm^-2
(assuming as spectral model a power law with photon index 1.4).
The AGILE-MCAL detector has a full solid angle acceptance, and is operational
in the range 0.4 - 100 MeV. The MCAL current trigger rate is on average ~ 0.15
every 20 sec.

Additional analysis of AGILE data is in progress.

The LIGO Scientific Collaboration and the Virgo Collaboration report:

In GCN 21656
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 , we reported the identification of the binary black hole
candidate G298936 in data from LIGO Hanford Observatory (H1) and LIGO
Livingston Observatory (L1). We have obtained and inspected the Virgo (V1)
data at the time of the candidate and found them to be suitable for manual
re-analysis using the PyCBC pipeline.

An updated BAYESTAR localization using data from all three detectors (H1,
L1, V1) is now available for retrieval from the GraceDB page:
bayestar-HLV.fits.gz. This is the preferred sky map at this time.

Compared to the initial BAYESTAR localization, the 50% area has decreased
from 605 to 277 deg2 and the 90% area has decreased from 2145 to 1219
deg2. The all-sky, marginalized luminosity distance estimate is 1541 +/-
415 Mpc (a posteriori mean +/- standard deviation), largely consistent
with the original estimate.

We will continue to refine the sky map using offline parameter estimation.

M. Cardillo (INAF/IAPS), F. Verrecchia (SSDC and INAF/OAR), M. Pilia
(INAF/OA-Cagliari), A. Bulgarelli (INAF/IASF-Bo), M. Tavani (INAF/IAPS,
and Univ. Roma Tor Vergata), C. Pittori (SSDC and INAF/OAR), G. Piano
(INAF/IAPS), A. Ursi(INAF/IAPS), I. Donnarumma (ASI), F. Longo (Univ.
Trieste and INFN Trieste), G. Minervini, A. Argan, Y. Evangelista
(INAF/IAPS), F. Lucarelli (SSDC and INAF/OAR), N. Parmiggiani, A. Zoli,
V. Fioretti, F. Fuschino (INAF/IASF-Bo), A. Trois (INAF/OA-Cagliari) M.
Marisaldi (INAF/IASF-Bo and Bergen University), A. Giuliani (INAF/IASF-Mi),
report on behalf of the AGILE Team:

In response to the LIGO/Virgo GW event G298936 (T0 = 2017-08-23 13:13:58.518
UTC; GCN #21656
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 ), analysis of AGILE data shows that the satellite at T0 was
in the South Atlantic Anomaly (SAA). Scientific telemetry was inhibited
during the time interval (T0 -300 s; T0 + 400 s).

We performed a preliminary analysis of the AGILE Gamma-Ray Imaging Detector
(GRID) in the first interval available after the SAA, between T0 +400s and
T0 +500s. In this time interval, the GRID exposure covered nearly 55% of the
LIGO/Virgo updated (GCN #21661
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 ) localization region (LR), observed at off-axis
angles between 5 deg and 50 deg.

An analysis of the data in the energy range 50 MeV - 10 GeV in this
integration time was performed and preliminary 3-sigma upper limits (UL)
values within the accessible LIGO/Virgo LR are:
from 3.1e-0.8 to 7.6e-7 erg cm^-2 s^-1 for integration time of 100s.

These measurements were obtained with AGILE observing a large portion of
the sky in spinning mode. Additional analysis of AGILE data is in progress

D. M. Palmer (LANL), S. D. Barthelmy (NASA/GSFC), A. Y. Lien (GSFC/UMBC),
T. Sakamoto (AGU), A. A. Breeveld (MSSL-UCL), A. P. Beardmore (U. Leicester),
D. N. Burrows (PSU), S. Campana (INAF-OAB), S. B. Cenko (NASA/GSFC),
G. Cusumano (INAF-IASF PA), A. D'Ai (INAF-IASFPA), P. D'Avanzo (INAF-OAB),
V. D'Elia(ASDC), S. Emery (UCL-MSSL), P. A. Evans (U. Leicester), 
P. Giommi (ASI), C. Gronwall (PSU), J. A. Kennea (PSU), 
H. A. Krimm (CRESST/GSFC/USRA), N. P. M. Kuin (UCL-MSSL), 
F. E. Marshall (NASA/GSFC), A. Melandri (INAF-OAB), 
B. Mingo (U. Leicester), J. A. Nousek (PSU), S. R. Oates (Uni. of Warwick), 
P. T. O'Brien (U. Leicester), J. P. Osborne (U. Leicester), 
C. Pagani (U. Leicester), K. L. Page (U.Leicester), M. Perri (ASDC), 
J. L. Racusin (NASA/GSFC), B. Sbarufatti (INAF-OAB/PSU),
M. H. Siegel (PSU), G. Tagliaferri (INAF-OAB), E. Troja (NASA/GSFC/UMCP)
report on behalf of the Swift team:

We report the search results in the BAT data within T0 +/- 100 s of the
LVC event G298936 (LIGO/VIRGO Collaboration GCN Circ. 21656
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 ), 
where T0 is the LVC trigger time (2017-08-23T13:13:58.518 UTC).

The center of the BAT FOV at T0 is
RA = 197.526 deg,
DEC = -23.362 deg,
ROLL = 292.808 deg.
The BAT Field of View (>10% partial coding) covers 9.44% of the integrated
LVC localization probability.

Within T0 +/- 100 s, no significant detections (signal-to-noise ratio >~ 5 sigma) 
are found in the BAT raw light curves with time bins of 1 s and 1.6 s. There is 
a 5.1 sigma signal at ~T-30 s in the 64-ms binned raw light curve. However, because 
no significant detections are found in the image domain, we believe this spike is 
unlikely to be an astrophysical source in the BAT FOV. Assuming an on-axis (100% 
coded) short GRB with a typical spectrum in the BAT energy range (i.e., a simple 
power-law model with a power-law index of -1.32, Lien & Sakamoto et al. 2016), 
the 5-sigma upper limit in the 1-s binned light curve corresponds to a flux upper 
limit (15-350 keV) of ~ 7.74 x 10^-8 erg/s/cm^2.

There are event data available from T+46.9 s to T+50.0 s. No detections above 
~ 5 sigma are found in the event data analysis.

BAT retains decreased, but significant, sensitivity to rate increases for
gamma-ray events outside of its FOV. About 38.31% of the integrated LVC
localization probability was outside of the BAT FOV but above the Earth's limb 
from Swift's location, and the corresponding flux upper limits for this region 
are within roughly an order of magnitude of those within the FOV.

D. Kocevski (NASA/GSFC), G. Vianello (Stanford), N. Omodei (Stanford), M. Razzano (Univ. Pisa & INFN Pisa) and S. Buson (NASA/GSFC) report on behalf of the Fermi-LAT team: 

We searched data collected by the Fermi Large Area Telescope (LAT) for a possible high-energy (E > 100 MeV) gamma-ray emission in spatial/temporal coincidence with the LIGO/Virgo trigger G298936 (GCN #21656
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 ). At the time of the trigger (T0 = 2017-08-23 13:13:58.520 UTC , 525186843.52 MET), the Fermi gamma-ray space telescope was within the South Atlantic Anomaly (SAA).  During SAA passages the LAT does not collect data due to the high charged particle background in this region. The LAT resumed data taking upon exiting the SAA at roughly T0 + 849 s.  At that time 47% of the Bayestar probability map based on H1, L1 and V1 detectors (GCN #21661
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 ) was in the LAT field of view, and we reached 100% cumulative coverage within 4434 s after the trigger. We define "instantaneous coverage" as the integral over the region of the LIGO/Virgo probability map that is within the LAT field of view at a given time, and "cumulative coverage" as the integral of the instantaneous coverage over time. 

We performed a search for a transient counterpart within the 90% contour of the LIGO/Virgo probability map in the time window from T0 to T0 + 10 ks, and no significant new sources are found. We also performed a search which adapted the time interval of the analysis to the exposure of each region of the sky.  No significant candidate counterpart was found.

The Fermi-LAT point of contact for this event is Daniel Kocevski (daniel.kocevski@nasa.gov) 

The Fermi-LAT is a pair conversion telescope designed to cover the energy band from 20 MeV to greater than 300 GeV. It is the product of an international collaboration between NASA and DOE in the U.S. and many scientific institutions across France, Italy, Japan and Sweden.

D. Xu (NAO/CAS), B. Li, H.B. Zhao, G.T. Zhaori, H. Lu, R.Q. Hong, L.F. 
Hu (PMO/CAS), H.X. Feng, Z.P. Zhu, T.M. Zhang, X. Zhou (NAO/CAS), J.Z. 
Liu, H.B. Niu, Y. Zhang, X. Zhang, G.X. Pu, S.G. Ma,T.Z. Yang, F.F. Song 
(XAO/CAS), J. Mao, J.M. Bai (YNAO/CAS) report on behalf of the 
Gravitational Wave Follow-Up Network by NAO-PMO-XAO-YNAO in China (GWFUNC):

We have performed tiled observations of LIGO/Virgo G298936 (LVC, GCN 
21656
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 ) using the 1-m Chinese Near Earth Object Survey Telescope (CNEOST) 
at Xuyi, Jiangsu, China.

CNEOST has a FOV of 3.0x3.0 deg^2, and carried out observations during 
17:46:32 - 20:45:48 UT on 2017-08-23 in the Sloan i-filter. In total, 
387 deg^2 is covered on the skymap with the highest localization 
probability in the north hemisphere. During the observations, the 
typical 5\sigma limit is ~19.8 mag.

Preliminary analysis reveals an optical transient (OT), named as 
GWFUNC-17ure, at the coordinates

RA (J2000) =     03:40:39.45
DEC (J2000) = +40:33:22.90

with m(i)~19.2 at 18:54:42 UT on 2017-08-23, apparently localized in its 
host galaxy SDSS J034039.45+403322.6 that has m(g)=20.32, m(r)=19.53, 
m(i)=19.14, as well as a PhotZ=0.12+/-0.04 (corresponding to D_L ~ 577 
+/- 200 Mpc).

Spectral observations of the above OT is encouraged.

S. Sugita, N. Kawai (Tokyo Tech), M. Serino (RIKEN), H. Negoro (Nihon U.),
S. Ueno, H. Tomida, M. Ishikawa, Y. Sugawara, N.Isobe, R. Shimomukai (JAXA),
T. Mihara, M. Sugizaki, S. Nakahira, W. Iwakiri, M. Shidatsu, M.
Matsuoka (RIKEN),
T. Yoshii, Y. Tachibana, S. Harita, Y. Muraki, K. Morita (Tokyo Tech),
A. Yoshida, T. Sakamoto, Y. Kawakubo, Y. Kitaoka, T. Hashimoto (AGU),
H. Tsunemi, T. Yoneyama (Osaka U.),
M. Nakajima, T. Kawase, A. Sakamaki (Nihon U.),
Y. Ueda, T. Hori, A. Tanimoto, S. Oda (Kyoto U.),
Y. Tsuboi, Y. Nakamura, R. Sasaki (Chuo U.),
M. Yamauchi, C. Hanyu, K, Hidaka (Miyazaki U.),
T. Kawamuro (NAOJ),
K. Yamaoka (Nagoya U.)
report on behalf of the MAXI team:

We examined the MAXI/GSC all-sky X-ray images (2-20 keV) obtained
in the orbit and the day after the LVC trigger
G298936 at 2017-08-23 13:13:58.518 UTC (GCN 21656
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 ).

GSC scanned more than 76% of the whole sky in the 92-min orbit,
which includes 81.0% of the 90% region in the bayestar skymap
scanned from the trigger time.
One day image obtained between 8-23 13:13:58 and 08-24 12:49 UTC
covers 100% of the 90% region in the bayestar skymap.
No significant new source was found in these images
and the 2-20 keV averaged photon counts in the skymap region is 0.26 counts/sec.
The 1-sigma (3-sigma) averaged upper limits
obtained from the one-scan and one-day images in the bayestar skymap are
12(36) and 2(6) mCrab, respectively
(see Serino et al., 2017 for the upper limit calculation).

If you require information of X-ray flux by MAXI/GSC at specific coordinates,
please contact the submitter of this circular by email.

M. Boer, R. Laugier (CNRS/ARTEMIS/UCA/OCA), A. Klotz (CNRS/IRAP/UPS), K.

Noysena (ARTEMIS/IRAP/CNRS/UPS), report on behalf the TZAC collaboration:

We have observed the enhanced localization of the LIGO/Virgo event
G298936 during the nights of 2017-08-23 and 2017-08-24 from 20h28UT
(observations are continuing).

8 fields covering about 40 sq. deg. have been scheduled on TRE and TCA
(see list below). More than 300 images have been taken at this time and
are under examination.

List of the field centres:

Instr       RA (HMS)    dec (DMS)    First scheduled at:
TCA        04:22:47    +49:57:53    2017-08-23T20:28:11.560
TCA        04:34:21    +49:57:53    2017-08-23T20:56:41.560
TCA        04:27:11    +48:06:17    2017-08-23T21:01:01.560
TCA        04:16:02    +48:06:17    2017-08-23T20:41:11.560
TCA        04:30:06    +51:49:29    2017-08-23T20:21:21.560
TRE        03:44:40    +38:06:00    2017-08-23T22:56:57.528
TRE        03:30:19    +33:54:00    2017-08-23T22:07:27.528
TRE        03:19:44    +29:42:00    2017-08-23T21:17:57.528
TRE        03:23:19    +38:06:00    2017-08-23T22:20:27.528
TRE        16:20:08    -24:53:59   2017-08-23T17:46:57.528
TRE        16:00:26    -20:41:59   2017-08-23T17:53:27.528
TRE        03:50:34    +33:54:00    2017-08-23T22:39:57.528
TRE        04:06:01    +38:06:00    2017-08-23T22:46:27.528

Note: TCA TAROT-Calern, field 1.9 sq. deg, TRE TAROT-R��union, field 4sq.deg.

I. A. Steele, C.M. Copperwheat, A.S. Piascik (Liverpool JMU)

report on behalf of

D.Bersier (LJMU), M.Bode (LJMU), C.Collins (LJMU), M.Darnley (LJMU), D.Galloway (Monash), A.Gomboc (Nova Gorica), S.Kobayashi (LJMU), A. Levan (Warwick), P.Mazzali (LJMU), C.Mundell (Bath), E.Pian (Pisa), D. Pollacco (Warwick), D. Steeghs (Warwick), N.Tanvir (Leicester), K. Ulaczyk (Warwick), K.Wiersema (Leicester)

and the GROWTH (Global Relay of Observatories Watching Transients Happen) collaboration.

----

We obtained a spectrum using the SPRAT spectrograph of the Liverpool
Telescope, La Palma on 2017-08-25 at 05:25UT of the CNEOST detected transient
GWFUNC17-ure (Xu et al, GCN 21675
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 ).

Using SNID we classify the source as a Type Ia supernova at redshift z=0.04
and age +18 days.

Assuming z=0.04, the Si ejecta velocity is ~10,000 km/s.

DisclaimerNone








--
iain Steele
Sent with Airmail

Y. Asaoka (Waseda U), A. Yoshida, T. Sakamoto, Y. Kawakubo, M. Moriyama,
Y. Yamada, A. Tezuka, S. Matsukawa (AGU), K. Yamaoka (Nagoya U),
S. Nakahira (RIKEN), I. Takahashi (IPMU), S. Ozawa, S. Torii (Waseda U),
Y. Shimizu, T. Tamura (Kanagawa U), W. Ishizaki (ICRR), M. L. Cherry (LSU),
S. Ricciarini (U of Florence), A. V. Penacchioni, P. S. Marrocchesi (U of Siena)
and the CALET collaboration:

The CALET Gamma-ray Burst Monitor (CGBM) was operating at the trigger time
of G298936 (The LIGO Scientific Collaboration and Virgo Collaboration,
GCN Circ. 21656
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 ).  No CGBM on-board trigger occurred at the time
of the event.  Based on the LIGO-Virgo localization sky map (bayestar-HLV.fits.gz),
the part of the northern arc of the high probability area was in the field-of-view of
CGBM.  The summed LIGO probabilities inside the HXM and the SGM field of view
are 57% and 66%.

Based on the analysis of the light curve data with 0.125 sec time resolution
from -60 sec to 60 sec from the trigger time, we found no significant excess
around the trigger time in either the HXM (7-3000 keV) or the SGM (40 keV -28 MeV)
data.

The CALET Calorimeter (CAL) was operating in high energy trigger mode at the trigger
time of G298936.  Similar to CGBM, the northern arc of the high probability area of the
LIGO-Virgo localization map was in the field of view of CAL.  Using CAL data, we have
searched for gamma-ray events in the 10-100 GeV band from -60 sec to +60 sec from
the GW trigger time and found no candidates.  The 90% upper limit of CAL is
2e-5 erg/cm2/s (10-100 GeV) when the summed LIGO-Virgo probabilities reaches at 20%.

Arvind Balasubramanian (IISER Pune), Varun Bhalerao (IUCAA), Dipankar Bhattacharya (IUCAA), Sukanta Bose (IUCAA), Gulab Chand Dewangan (IUCAA), Ranjeev Misra (IUCAA), Sanjit Mitra (IUCAA), A R Rao (TIFR), Tarun Souradeep (IUCAA), Santosh Vadawale (PRL), on behalf of the Astrosat CZTI team report:

We carried out offline analysis of data from Astrosat CZTI in a 100 second window centred on the G298936 trigger time, 2017-08-23 13:13:58.518 UTC, to look for any coincident hard X-ray flash. CZTI is a coded aperture mask instrument that has considerable effective area for about 29% of the entire sky, but is also sensitive to brighter transients from the entire sky. Based on the pointing direction of Astrosat at the time of the GW event and the Bayestar skymap provided by LVC (bayestar-HLV.fits.gz,0), the sky visible to CZTI has 69.2% probability of containing the EM counterpart, of which only 0.6% is in the favourable region while the rest is obscured by satellite elements.

CZTI data were de-trended to remove orbit-wise background variation. We then searched data from the four independent, identical quadrants to look for coincident spikes in the count rates. Searches were undertaken by binning the data in 0.1s, 1s and 10s respectively. Statistical fluctuations in count rates were estimated by using data from 10 neighbouring orbits. We selected confidence levels such that the probability of a false trigger in this 100s window is 10^-4. We do not find any evidence for any hard X-ray transient in this window. 

We convert our count rates into fluence and flux limits by assuming that the source spectrum has band parameters alpha = -1.0, beta = -2.5, Epeak = 300. We use a detailed mass model of the satellite to calculate the instrument response in the direction having the maximum probability density in the HLV map. We then assume that the source is modeled by a band function with parameters alpha = -1.0, beta = -2.5, Epeak = 300 keV, and get the following upper limits for source fluence and flux in the 20-200 keV band:

0.1 s: Effective fluence limit= 5e-6 ergs/cm^2; flux= 5e-5 ergs/cm^2/s
1.0 s: Effective fluence limit= 1e-5 ergs/cm^2; flux= 1e-5 ergs/cm^2/s
10.0s: Effective fluence limit= 1e-5 ergs/cm^2; flux= 1e-6 ergs/cm^2/s

Flux limits over the rest of the northern localisation lobe are similar in magnitude.

CZTI is built by a TIFR-led consortium of institutes across India, including VSSC, ISAC, IUCAA, SAC and PRL. The Indian Space Research Organisation funded, managed and facilitated the project.

I. Martinez-Castellanos (University of Maryland, College Park) and
A.J. Smith (University of Maryland, College Park) on behalf of the
HAWC Collaboration:

HAWC was operating and our real-time all-sky GRB monitoring analysis was
running at the time of the G298936 event. At the time of the event, the
HAWC detector was oriented at (��, ��) = (165.9��, 19.0��), local zenith. 60%
of the GW candidate location probability contour fell within our observable
field (0-45 deg zenith angle), including the location with highest
probability.

We perform a real-time search for counts above the steady-state cosmic-ray
background using 4 sliding time windows (0.1, 1, 10, and 100 seconds)
shifted forward in time by 10% of their width over the course of the entire
observing period. Within each time window, we search the HAWC sky within 45
degrees of zenith using 2.1 deg x 2.1 deg square bins shifted by ~0.1 deg
along the directions of Right Ascension and Declination. This analysis is
optimized for detecting ~100 GeV photons and is sensitive to the most
fluent GRBs. It did not report any significant post-trials events near the
time of the LIGO trigger.

After the LIGO trigger was reported, we re-analyzed the data collected
around the trigger time (T0-dt to T0+3dt) on 3 timescales (dt = 1, 10, 100
sec) with a reduced threshold to account for the reduced number of trials.
No significant candidates were identified.

Additionally, we searched for longer duration emission, integrating from
the time of the trigger to the time when the overlapping 90% containment
LIGO contour left our FOV. For reference, the maximum probability location
(RA = 66.7 deg, Dec = 49.7 deg) was observed from T0 to T0 + 8 ks. This
search covered the same 60% of the probability space as the prompt search.
No significant excess was observed. HAWC has a 5-sigma point-source
sensitivity to a flux >1 TeV of ~2x10^-11/cm^2/s, about ~1 ���crab unit���, for
a single transit observation.

HAWC is a TeV gamma ray water Cherenkov array located in the state of
Puebla, Mexico that monitors 2/3 of the sky every day with an instantaneous
field-of-view of ~2 sr.

The LIGO Scientific Collaboration and the Virgo Collaboration report:

We have re-analyzed data from LIGO Hanford Observatory (H), LIGO
Livingston Observatory (L), and Virgo (V) around the time of the binary
black hole merger candidate G298936 (GCNs 21656
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  and 21661
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 ) taking into
account our current understanding of calibration uncertainties.

Parameter estimation has been performed using LALInference (Veitch et al.,
PRD 91, 042003) and a new sky map, LALInference_r1.fits.gz, can be
retrieved from the GraceDB event page:

https://gracedb.ligo.org/events/G298936

This is the preferred sky map at this time.

The new localization is consistent with but slightly smaller than the
BAYESTAR localization using HLV data (GCN 21661
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 , bayestar-HLV.fits.gz).
The 50% area has decreased from 277 to 207 deg2 and the 90% area has
decreased from 1219 to 952 deg2. The all-sky, marginalized luminosity
distance estimate is 1738 +/- 477 Mpc (a posteriori mean +/- standard
deviation), largely consistent with the original estimate.

D. Roberts, U. Kolb & M.Morrell (The Open University) reporting on behalf of the PIRATE group:


We observed 5 separate fields containing the top 6.66% probability area of the bayestar-HLV skymap of the LIGO/Virgo candidate G298936 using our 0.43m robotic telescope at Teide Observatory, Tenerife, Spain. We only acquired 38 images due to a subsequent alert 2 days later that contained a much higher EM counterpart probability, all images were obtained using the R filter and 100s exposure length. Initial observations began at 2017-08-23T02:09:23, approximately 9 hours after the updated GCN alert was received and 13hrs after the initial trigger. A detailed list of all the fields observed can be found on GraceDB under the EMObservations tab. Further observations are planned.


-- The Open University is incorporated by Royal Charter (RC 000391), an exempt charity in England & Wales and a charity registered in Scotland (SC 038302). The Open University is authorised and regulated by the Financial Conduct Authority in relation to its secondary activity of credit broking.

D. Svinkin, S. Golenetskii, R.Aptekar, D. Frederiks, P. Oleynik,
M. Ulanov, A. Tsvetkova, A. Lysenko, A. Kozlova, and T. Cline,
on behalf of the Konus-Wind team, report:

Konus-Wind (KW) was observing the whole sky at the time of the
LIGO/Virgo event G298936 (2017-08-23 13:13:58.522 UTC, hereafter T0;
LIGO/Virgo Collaboration GCN Circ. 21656
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 ).

No triggered KW event happened from ~0.6 days before and up to ~1.8 days
after T0. The closest waiting-mode event was ~2.5 hours before T0.
Using waiting-mode data within the interval T0 +/- 100 s,
we found no significant (> 5 sigma) excess over the background
in both KW detectors on temporal scales from 2.944 s to 100 s.

We estimate an upper limit (90% conf.) on the 10 keV ��� 10 MeV fluence
to 8.8x10^-7 erg/cm^2 for a burst lasting less than 2.944 s and having a
typical KW short GRB spectrum (an exponentially cut off power law with
alpha=-0.5 and Ep=500 keV). For a typical long GRB spectrum (the Band
function with alpha=-1, beta=-2.5, and Ep=300 keV), the corresponding
limiting peak flux is 2.9x10^-7 erg/cm^2/s (10 keV - 10 MeV, 2.944 s scale).

All the quoted values are preliminary.

A. M. Read (U. Leicester), A. Tiengo (IUSS Pavia), R. Salvaterra
(INAF-IASF Milano), E. Troja (NASA/GSFC), and R. D. Saxton (ESAC)
report:

We analyzed the XMM-Newton slews made after the LIGO/Virgo G298936/
GW170823 event. The first three slews intercepting the GW localization
map cover it for about 23.5 square degrees:

Obs ID     |    date    |   T-T_GW   |coverage of localization map
9324900002 | 2017-09-05 |   13.1 d   |  4.2 deg2
9325000003 | 2017-09-07 |   14.7 d   |  8.7 deg2
9325100002 | 2017-09-09 |   16.7 d   |  10.6 deg2

For each dataset (EPIC pn data with the Medium filter) we performed
the source detection following the method described in Troja et al. 2016
(ApJ, 822, L8). Typical sensitivity limits of slew observations are
~6e-13 ergs cm-2 s-1 in the 0.2-2 keV band and ~4e-12 ergs cm-2 s-1
in the 2-12 keV band.
The list of the most significant 0.2-12 keV band detections (DET_ML>12)
in each slew intersecting the GW170823 localization map, with no
counterpart within 30 arcsec in the ROSAT All Sky Survey (Boller et al.
2016, A&A 588, A103) and with no X-ray sources with compatible position
and flux detected in archival XMM-Newton observations, is reported below.
The flux and counts are computed in the 0.2-12 keV energy band.Rank (1-4,
1 most likely, 4 least likely) indicates how likely the object is to be
the GW counterpart.

----------
Revolution 3250, Observation ID 9325000003
RA (deg)   DEC (deg)  CTS  DET_ML EXPOSURE(s) FLUX(ergs cm-2 s-1) Rank
259.7674   -38.478    6.0   13.3    6.2        9.7e-12              4
----------
Revolution 3251, Observation ID 9325100002
RA (deg)   DEC (deg)  CTS  DET_ML EXPOSURE(s) FLUX(ergs cm-2 s-1) Rank
267.8741  -46.1932    5.2   17.3    7.0        7.4e-12              3
264.2881  -40.5070    4.6   12.1    5.6        8.2e-12              4

Dong Xu (NAOC), Evangelos-Dimitrios Paspaliaris, Sarel Jacobus van der 
Walt, Margerita Dahl, Sarah Savic Kallesoe, Zlatko Saldic, Markus Thor 
Rasmussen, Lasse Stampe Frolich, Simon Refshauge Papst, Martin Jan 
Sandberg, Johan P. U. Fynbo (NBI/UCPH), Anne Noer Kolborg (DTU), Frank 
Grundahl (Aarhus U.), Elena Pian (Bologna) report:

We obtained a spectrum of GWFUNC-17ure (Xu et al., LVC GCN 21675
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 ), an 
optical transient in the credible region of G298936 (LIGO/Virgo 
Collaboration, LVC GCN 21656
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 ), starting at 02:22:02.83 UT on 2017-08-25 
using the 2.56-m Nordic Optical Telescope (NOT) equipped with the ALFOSC 
imaging spectrograph.

Fitting to the supernova templates using SNID, the source is classified 
as a SN Ia at z=0.041 about 19 days after maximum, which indicates that 
this source is unrelated to the G298936 event. Our classification is 
consistent with the result in Steele et al. (LVC GCN 21691
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 ).